This invention relates to electromechanical devices such as alternators, generators and the like and especially to hybrid devices that combine the features of field controlled-type alternators and permanent-magnet-type alternators. More particularly, the invention relates to a hybrid-type alternator that utilizes a stationary field coil to control the voltage output as well as a permanent-magnet-type arrangement.
Permanent-magnet-type alternators are used in a variety of applications including aircraft engine systems. While these devices have many advantages including light weight and compact construction, they do have some practical limitations due to the direct relationship between voltage and rotor speed. In other words, the open circuit voltage is directly proportional to the rotational speed of the device. The maximum output voltage may be an important design consideration for permanent-magnet-type alternators in view of certain factors such as corona, insulation, altitude requirements, connector insulation resistance and the maximum voltage capability of the load.
The life and integrity of semi-conductor control electronics, are especially vulnerable to voltage levels beyond their capabilities. The subsequent limitation on the maximum output voltage from the alternator can preclude the use of permanent-magnet-type devices in some applications where a large speed ratio is required.
One solution to the problems outlined above, is to utilize a field-controlled alternator to overcome the high voltage problem of the permanent-magnet-type alternator. A typical construction of such a device includes a wound rotor, where the rotor field is regulated externally by a control circuit that senses and controls the alternator output voltage. The field power is supplied to the rotor by an "inside-out" (stationary field, rotating armature) alternator through a rotating rectifier assembly. Both of these components are typically on the same shaft and in the same package as the main rotating-field alternator itself. A small permanent magnet alternator section is included in the device to provide the initial field power to get the main alternator started.
While this type of alternator provides a means for limiting the output voltage at high speeds, it has certain disadvantages as well. Most importantly, placing windings on the rotor is often unacceptable due to mechanical stresses. This makes the device vulnerable to failure under certain conditions.
An alternative to the wound rotor, field controlled alternator is a stationary-field controlled permanent magnet alternator. This configuration would have the voltage control feature of the traditional field-controlled device, but also would provide the inherent reliability of a permanent magnet alternator. The rotor construction would be similar to that in a permanent magnet alternator, and there would be no windings on the rotor and no rectifier assembly. The magnets would provide a self-starting feature for the alternator that would eliminate the need for the field power alternator in the traditional device. An additional advantage of the field controlled, permanent magnet alternator is that if the field power failed, there would still be power supplied due to the permanent magnets in the rotor.
One particular type of field-controlled permanent magnet alternator is referred to as a hybrid homopolar alternator. This type of device has a permanent magnet section on the shaft which may share the same stator as the homopolar alternator. In this type of construction, the permanent magnet section provides both the starting flux for the homopolar section, as well as some output power to the load. Prior art devices of this type, however, have been complex and cumbersome. This makes them unsuitable for certain applications including aircraft engine systems.
The device of the present invention, however, resolves the difficulties referred to above and affords other features and advantages heretofore not obtainable.